1. Field of the Invention
This invention relates to a torque sensor for detecting the torque of a rotating shaft, more particularly to a torque sensor suitable for measuring the torque of a drive shaft, steering shaft or other rotating shaft of an automobile and the like.
2. Description of the Prior Art
One torque sensor which measures the torque of a shaft by sensing the amount of twist therein is unable to measure static torque. Because of this inadequacy, there have recently been introduced a number of torque sensors operating on the principle of magnetostriction. One example of such a torque sensor is described in Japanese laid-open patent publication No. 57(1982)-211030, wherein a ribbon-like magnetostrictive strip is wound on a shaft whose torque is to be measured.
The structure of the torque sensor disclosed by this publication requires that the magnetostrictive member be fixed directly on the shaft whose torque is to be measured so that the shaft itself becomes one component of the torque sensor. This is disadvantageous for several reasons. First, during manufacture of the torque sensor, it is generally necessary to attach the magnetostrictive member to a shaft of considerable length such as an automobile drive shaft, and this is difficult to do with high positional precision. Then, after the magnetostrictive member has been fixed on the shaft and up to the time that the shaft is installed in the vehicle, which is generally late in the assembly process, it is necessary to take great care in transporting and storing the shaft bearing the magnetostrictive member so as to protect the member from damage and adherence of dust or the like. The need to take these precautions greatly complicates the overall process of shaft installation.
Moreover, since the shaft whose torque is to be measured is involved as one component of the sensor, the sensor cannot be completed without mounting the other components on the shaft. As a result, it is not possible to adjust the gap between the magnetostrictive member and the associated coils until the assembly is carried out. Another disadvantage arises from the fact that drive shafts and other such automotive parts are only required to have adequate strength and are not required to have high dimensional precision. It therefore becomes necessary to use a special, separate adjustment means for adjusting the gap, which leads to further inconveniences as regards inventory control, performance control, maintenance and the like.
Also, since the structure is such that the coil and other components which are relatively susceptible to damage by mechanical shock are not capable of being easily removed from the exterior, special care has to be used during assembly and installation. This structure would degrade sensing accuracy and is also disadvantageous from the standpoint of maintenance.